High-frequency transformer



June 28, 1949. EARLY ET AL 2,474,395

HIGH-FREQUENCY TRANSFORMER Filed Sept. 20, 1945 attorney.

Patented June 28, 1949 HIGH-FREQUENCY TRANSFORMER Harold 0. Early,Beaverton, and William G. Dow

and Robert Ohlsson, Ann Arbor, Mich., assignors to General MotorsCorporation, Detroit,

Mich., a corporation of Delaware Application September 20, 1945, SerialNo. 617,614

Claims. 1

This invention relates to electrical transformers and more specificallyto electrical transformers for use with high frequency electrical power.It is of course well known that the problems attendant transformerdesign are radically differ" ent when the same is to be used on a highfrequency power installation than when the same is to be used onordinary commercial power which is usually of a frequency of 60 cycles.As the frequency increases the problems of iron and copper loss varyconsiderably, the hysteresis and eddy current factors of the former takeon added importance and the skin effect of high frequency current variesthe copper consideration. Therefore in proceeding from commercial lowfrequencies to for example frequencies in the radio broadcast band anentirely different set of considerations for transformer design arefound. The transformers usually are air core though in some instancesmay be provided with a coznminuted or laminated iron core which isformed of small particles isolated from each other. in radio equipmenthowever the amount of current and energy being handled is relativelysmall.

There has recently been considerable activity in the fields of highfrequency heating and weld ing and in such a system the amount ofcurrent utilized is relatively large and the previouslymentioned highfrequency transformers incapable of handling the load. It was thereforenecessary to design transformer means to supply the heavy currentheating and welding equipment at advanced frequencies.

It is therefore an object of our invention to provide transformer meansfor operation at frequencies and very high currents.

It is a further object of our invention to provide a high frequencytransformer for heavy currents such for example as for welding.

It is a still further object of our invention to provide a design forhigh frequency heavy duty transformers in which the skin and proximityeffects of high frequency current are taken into consideration.

With these and other objects in view which will become apparent as thespecification proceeds, our invention will be best understood by refer"ence to the following specification and claims and the illustrations inthe accompanying drawings, in which:

Figure 1 is a perspective view of a transformer winding and terminalarrangements embodying our invention;

Figure 2 is a front elevation of a portion of the winding per se takenon the line 2-2 of Fig. 1;

Figure 3 is an enlarged end view of the winding and terminal plates,parts being broken away and shown in section to illustrate theconnection of the windings and plates; and

Figure 4 is an enlarged sectional View through a portion of the terminalplates.

There are a number of points in such design that must be taken intoConsideration for proper final construction. The first of these is thatsince high frequency current is used both the skin effect and the socalled proximity effect will be present in the conductors. The skineffect causes current to tend to flow along the surface of the conductorand the proximity effect makes the skin effect more pronounced in twoadjacent conductors in which the current is flowing in oppositedirections. In other words the current tends to pile up on the surfaceof a conductor adjacent another in which the current is flowing in theopposite direction. These effects can be used to advantage to decreaselosses by providing large surface areas between adjacent turns ofprimary and secondary, as by using fiat conductors, and yet the totalvolume of the winding will be less than if some other cross section ofconductors were used.

The second consideration is that the leakage flux must be very low forvarious reasons par ticularly to aid in keeping the size of thetransformer small. It has been pointed out above that the optimum crosssection for the conductor is flat from a high frequency standpoint andof course this could be manufactured by forming the windings. In orderto produce interlinked windings with a minimum leakage, edge winding isused with the primary and secondary turns completely interleaved. Thisprovides a very low leakage flux.

The third consideration is that the primary inductance must beadequately large in a high frequency transformer or the magnetizingcurrent will be excessive. To provide this the diameter of the windingsmust be large or the axial length short. By using the edge woundconstruction the axial length is kept to a minimum so that a smallerdiameter winding can be used than would otherwise be the case. Ingeneral, these principles apply to either air core or iron coretransformer construction and if iron core is used then following theseprinciples may result in economy.

While the above considerations relate to the windings per se a part alsorelates to the conductors connected to the windings and which provideconnection thereto. Flat parallel surare rigidly secured cooling finsfaces are used to prevent excessive current pil ing up and attendantlosses.

In welding transformers or others capable of handling high currents, theratio of primary to secondary turns is usually large and it is notunusual to have the secondary consist of plurality of single turns inparallel to carry heavy current. Such a transformer is illustrated inthe accompanying drawings but it is desired to state that this is merelyexemplary of one form of applicants invention and that the same could beapplied to transformers of different ratios.

The primary in the present illustration consists of a continuous helicalcoil 2 formed of a flat member shaped into a coil around its majordimension. The flat sides are coated or covered with any suitableinsulating means i to maintain the turns electrically isolated. Thehelical turns are stretched out or axially spaced far enough apart sothat a single secondary turn may be inserted between each primary turn,thus providing in general a continuous hecal primary winding andalternate single turn second-- ary coils between each primary turn.

The single secondary coils or turns are shown in Fig. 2 in which theprimary coil begins at the upper left hand corner of the figure and thenappears again as the first full turn and as each alternate turnthereafter. turn such as G, 8, ll], l2, etc., starts and ends atapproximately the center and the ends are offset so that the turn willfit between the primary turns. The adjacent ends of successive secondaryturns are oppositely notched as at so that they may overlap to a certaindegree but since the contour of each turn is helical the two ends of oneturn are not in juxtaposition but rather the ends of one turn overlapand are adjacent to the opposite ends of the turns on either sidethereof.

There are also rigidly secured to each end of each turn outwardlyprojecting flat conductor strips I5, I3, 213, 22, etc., that are twistedso that their outer ends lie in a horizontal plane, the two adjacentstrips connected to the ends of two coils being in spaced parallelrelation through.- out their surfaces to keep the current spread over alarge surface area by utilizing the skin and proximity effects of highfrequency cur rents and maintains the inductance loop as small aspossible. At several places around the periphery of each of thesecondary coils there In the present instance three of these are shown.Each fin has a central opening 25 therein, and tubes 23, through whichcoolant circulates, project through an aligned series of fins tocool thesame.

It will be noted with respect to Fig. 2 that the terminals H5, H1, 25!,etc., of the secondary turns are grouped into two parallel series oneabove the other. It is desired to provide as convenient terminalfacilities as possible for a o plication of the power of the transformerto a load and since as reviously mentioned this is for high frequencyinstallation the skin, proximity and inductive loop effects are takeninto consideration in electrode or output terminal design. For someapplications it is desirable to provide surfaces upon which pieces to bewelded can be rested and this type application is illustrated in thisinstance. The simplest manner in which this could be accomplished wouldbe to connect all of the terminal strips connected to one end Eachsecondary of each secondary turn to one plate and all of the others to asecond plate in close but spaced relation thereto. One plate would thenbe longer than the other to provide a step with the two differentelevations providing power output and upon which parts could be laid forwelding. This however is not mechanically feasible, for the terminalstrips when twisted would be too close together so two series atdifferent elevations are used.

All of the uppermost strips in the upper series Iii, 22, etc., areconductively connected to a large sheet 36 and all of the lower stripsof the upper series to a second sheet 32 which is insulated from thefirst sheet by an insulating layer 34. A third sheet 35 is connected tothe third series of contacts it, etc., and lies parallel to the secondsheet. The terminal strips connected to sheets 32 and '35 are so poledthat current will be flowing in the same direction in these two so theyare not insulated from each other. The lowest row of strips Zil, etc.,are connected to a bottom plate 33 which, as in the case of the two .topsheets, is insulated from the sheet 38 above it by a layer of insulatingmaterial 40.

Since therefore the polarity of the top and bottom sheets Eli and 38 isthe same these can be connected together and the ultimate result is twodeck plates or electrodes to which the Work can be applied. This isaccom'p ished by integral side panels 42 extending from sheet 38 tosheet 30 and insulated from the two intermediate sheets by a layer ofinsulation 44 as best shown in Fig. 4. The assembly of plates is securedtogether by suitable bolts it properly insulated from one set of sheetsor plates. Intermediate sheet 32 is extended beyond the remaining sheetsand it with the upper sheet form the work surfaces for application ofparts to be welded as shown at 48 and 58.

We therefore have provided a transformer for use on high frequencycurrent in which the inductance loops formed by the conducting portionsof the leads to the work are kept to a minimum, the skin effect andattraction of currents flowing in opposite directions has been fullyutilized to provide an efficient device, the primary inductance is highand the leakage flux low, and the size kept to a minimum.

We claim:

1. In transformer means for use with high frequency current, a primarywinding in the form of a helix with the turns axially separated, aplurality of single turn secondary windings interposed between theprimary turns to form a cylindrical member, the ends of one secondaryturn lying adjacent the ends of each secondary turn on opposite sidesthereof, conductor strips connected to the ends of each secondary turn,which will result in associated pairs of strips, each pair of stripslying in spaced parallel relation throughout their length and twistedintermediate their ends so that the planes of the opposite ends of eachstrip are at right angles toeach other and a plurality of parallelspaced conductive plates connected to the extending series of stripends.

2. In transformer means for use with high frequency current, a primarywinding in the form of a helix with the turns axially separated,- aplurality of single turn secondary windings interposed between theprimary turns to form a cylindrical member, the ends of I one secondaryturn lying adjacent the ends of each secondary turn on opposite sidesthereof, conductor strips connected to each end of each secondary turnforming associated pairs of strips, each pair of strips lying in spacedparallel relation throughout their length and twisted from a positionparallel to the faces of the turns to one at right angles thereto toform separate series and a plurality of parallel spaced conductiveplates each connected to a series of extending ends of the strips.

3. In transformer means for use with high frequency current, a primarywinding in the form' of a helix with the turns axially separated, aplurality of single turn secondary windings interposed between theprimary turns to form a cylindrical member, the ends of one secondaryturn lying adjacent the ends of each secondary turn on opposite sidesthereof, conductor strips connected to each end of each secondary turnresulting in a series of closely spaced parallel pairs, said stripsbeing twisted so that their extending ends are at right angles to theirinner ends but that they remain parallel to the other one of the pairthroughout their lengths to utilize the proximity efiect and a pluralityof conductive terminal plates connected to the extending ends.

4. In transformer means for use with high frequency current, a primarywinding in the form of a helix with the turns axially separated, aplurality of single turn secondary windings interposed between theprimary turns to form a cylindrical member, the ends of one secondaryturn lying adjacent the ends of each secondary turn on opposite sidesthereof, conductor strips connected to each end of each secondary turn,a plurality of superimposed plates of conductive material connected tothe conductor strips, insulating material separating a portion of theplates, conductive means interconnecting other of said plates so thattwo plates of opposite polarity are provided at the locale to supportthe work,

5. In transformer means for use with high frequency current a multi-turnprimary windin in the form of a helix with axial spacing between turns,a plurality of single turn secondary windings interposed one betweeneach pair of primary turns, conductor strips radially extending fromeach secondary terminus in parallel pairs, first conductive meansconnected to a portion of these strips to connect one end of eachsecondary turn, second conductive means connected to the remainingstrips to connect the opposite secondary ends, said first and secondconductive means being mechanically supported in major dimensionjuxtaposition but electrically insulated from each other to utilizethroughout the principle of proximity current flow to decrease loss.

HAROLD C. EARLY. WILLIAM G. DOW. ROBERT OHLSSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,394,044 Stephens Oct. 18, 19211,506,320 Northrup Aug. 26, 1924 1,880,199 Gebhard et al. Oct. 4, 19322,325,810 Strickland Aug. 3, 1943 2,378,884 Seifert June 19, 1945FOREIGN PATENTS Number Country Date 372,475 Germany Mar. 28, 1923

